This invention relates to cathode ray tubes and more particularly to a process for fabricating a black matrix screen structure having phosphor receiving areas smaller than a given exposure area effected by directing actinic radiation through an apertured mask.
Generally, matrix screen structures for cathode ray tubes are of either the positive or negative tolerance type. In the positive tolerance type screen structure the matrix holes for phosphor deposition are larger than the electron beam spot size of an operating tube. In the negative tolerance type screen structure the matrix holes for phosphor deposition are smaller than the electron beam spot size of an operating tube. Moreover, the positive tolerance type screen structure, which is the least popular structure, is normally fabricated by merely overexposing through the apertured mask associated with present-day "shadow-mask" type structures.
However, the more popular negative tolerance type structure, wherein the matrix holes for phosphor deposition as smaller than the electron beam spot size, are fabricated in accordance with several techniques. In one known process, the structure is exposed through the associated apertured mask to provide matrix holes of a given size and suitable for receiving phosphors. Thereafter, the apertures of the mask are enlarged to provide a beam spot size larger than the matrix holes wherein the phosphors are deposited.
In another known process, a film of photo-sensitive resist is exposed through the apertured mask which has had the apertures partially filled with a liner. Thereafter, the liner is removed to provide enlarged apertures and a beam spot size larger than the matrix holes. In another similar technique, a temporary mask is affixed to the regular apertured mask in order to reduce the aperture size. The matrix holes are exposed and the temporary mask is removed to provide enlarged mask apertures whereby the beam spot size is larger than the matrix holes.
Other known methods for providing negative tolerance structures include an acid etch back process wherein a photo-sensitive resist film is exposed through the normal apertured mask and the exposed film reduced by an acid treatment. Thus, matrix holes of a size smaller than the beam spot size are achieved. Also, underexposure or a "print down" technique is utilized wherein a photo-sensitive resist film is underexposed through the apertured mask and a portion of the underexposed film is washed away to leave a film spot size smaller than the size of an electron beam passing through the same apertured mask and impinging the screen.
Although each of the above-mentioned techniques has been or still is employed for fabricating screens for cathode ray tubes, it has been found that each leaves something to be desired. For example, enlarging the apertures of the mask, filling the apertures with a liner, and utilizing a temporary mask have all been found to be extremely cumbersome, expensive and not particularly satisfactory techniques for fabricating negative tolerance type screen structures. Also, it has been found that acid treatment techniques are most difficult to use due to the problems of size control of the exposed film. Moreover, under-exposure techniques have a tendency to cause deposition of insolubilized film which is relatively thin or which has a tendency to loosen and leave the support to which it is affixed.